Jk rowling, the woman who created harry potter, creating new magic tonight. But her fans had no idea until now. Here is nick schriffen. Reporter: Inside this london store, the shelves bend under the books of jk rowling. But robert galbraith? Do you think who robert is? I have no idea. I look and couldn't find his debut novel released in april. No galbraith. It hadn't really caused a blip on the radar, a large scale sales or interest. Reporter: In three months, cuckoo's calling sold 3 million. Today it sold 1 100,000. Why? She sold 450 million books. Spawned an $8 billion franchise. Jk rowling wanted to write unanimously so she could know what the reviewers really thought. It's very appealing the idea of writing under a different name, realistically, my chances of getting away with that are incredibly remote. She did get away with writing this book until the london journalist followed the clues. Same agent, same publisher, same editor. Crucially, you know, some of the same phrases. Similar style of writing. Can you show me are the harry potter books are. Today we found lots of rouling fans, even this 10-year-old, who now say they're a galbraith fan. Do you think you would want to read any jk rowling book? Yeah. Even one about a mystery? In the case of who the author is? Mystery solved. Nick schriffen, abc news, london.

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